US forces in Syria started pulling back Monday from Turkish border areas, opening the way for Ankara's threatened military invasion and heightening fears of a jihadist resurgence.
The withdrawal from key positions along Syria's northern border came after the White House said it would step aside to allow for a Turkish operation President Recep Tayyip Erdogan has warned could come at any moment.
The move marks a major shift in US policy, and effectively abandons the Kurds, who were Washington's main ally in the years-old battle against the so-called Islamic State group.
The Syrian Democratic Forces (SDF), the Kurdish-led militia that controls much of northeastern Syria, said early on Monday in a statement that "US forces withdrew from the border areas with Turkey".
The Syrian Observatory for Human Rights war monitor confirmed that US forces had pulled back from key positions in Ras al-Ain and Tal Abyad.
A Kurdish official also told AFP that US forces had started withdrawing from the frontier, making way for the Turkish onslaught, the scope of which remains unclear.
Turkey has sent reinforcements to the border in recent weeks, and Erdogan said Monday in televised remarks the long-threatened offensive could "come any night without warning".
His comments came after Foreign Minister Mevlut Cavusoglu said on Twitter that Turkey was "determined to ensure our country's existence and security by clearing terrorists from this region".
He was referring to the SDF, which has ties to Kurdish militants inside Turkey and which Ankara considers a terrorist organisation.
Fearing yet another chapter of bloodshed and mass displacement in the Syrian conflict, the United Nations said it was "preparing for the worst".
The European Union warned that civilians would once again bear the brunt of a military assault.
'AT ALL COSTS'
In its statement, the White House made clear it would stand aside when its NATO ally Turkey moves in.
SDF spokesman Mustefa Bali said Washington's decision "is about to ruin the trust and cooperation between the SDF and US".
"People here are owed an explanation," he said on Twitter.
The SDF, which spearheaded -- with backing from the US-led coalition -- several of the most significant battles against IS over the past five years, also vowed to resist any Turkish attack.
"As the Syrian Democratic Forces, we are determined to defend our land at all costs," it said in a statement posted on social media.
Ankara says it wants to urgently establish a "safe zone" on the other side of the border where it could send back some of the 3.6 million refugees who fled the eight-year war in Syria to live on Turkish soil.
But the Kurds argue that Turkey's goal is to weaken the Kurdish presence in the region by modifying the demographics of the area with the return of mostly Sunni Arab refugees.
Ankara's planned offensive is expected to focus on the border areas of Ras al-Ain and Tal Abyad, which are Arab-dominated towns governed by the Kurdish administration in northeast Syria.
Kurdish forces have dug trenches and tunnels in the two areas in preparation for a Turkish offensive, according to the Observatory.
'PREPARE FOR WAR'
In Ras al-Ain on Sunday, local armed forces allied with the Kurdish administration stood behind dirt berms, monitoring the frontier.
They had covered streets in the town with large metal canopies to block the view of Turkish drones, sources there told AFP.
"The prudent should prepare for war," said Mustefa Bozan, a 79-year-old shopkeeper.
Nearby, Issam Daoud said that local security forces have erected checkpoints at key entrances to the town.
"The fate of the region will be the same as that of Afrin," the 38-year-old said, referring to a former Kurdish enclave captured by Turkish troops and Syrian rebels last year.
The SDF has also warned that a Turkish offensive would reverse the military gains achieved against IS and allow for the jihadist group's surviving leaders to come out of hiding.
In its statement, it said that IS cells would break out detained jihadists from Kurdish prisons and take over camps where their relatives are held.
But Turkey's presidential spokesman Ibrahim Kalin said Monday that Ankara "will also continue to fight against DAESH (IS) and will not allow it to return in any shape and form."
While a Kurdish-led operation earlier this year saw the death of IS's territorial caliphate, the organisation isn't dead and sleeper cells have been active in several parts of Syria and Iraq.
The US itself has warned that, short of sustained international military pressure on the remnants of the jihadist group, IS would soon have the ability to regroup and reclaim some territorial control in the region.